Typically a thick film dielectric is composed of fine particle glass mixed with ceramic oxide particulate fillers suspended in an organic vehicle suitable for screen printing. The filler is mixed with glass to optimize a broad set of property requirements which typically involve increasing fracture toughness and strength. Most fillers also have an effect on the optical properties of the dielectric because they have refractive indexes which are sufficiently different from the glass so that they will act to scatter light. However, light scattering alone is normally not sufficient to provide laser trim characteristics desirable for a multilayer thick film dielectric.
Two principal features of the dielectric are important for desired performance. The first characteristic is the ability to laser trim resistors, cutting through the resistor material, and continuing into the dielectric below. A suitable dielectric provides sufficient strength and light scattering to diffuse the localized laser energy so that brittle cracking or chipping of the dielectric are avoided. A dielectric which is prone to cracking will continue to propagate a crack which was initiated during trimming. This type of minimal performance, typical of many commercial dielectrics, suffers from inadequate performance for protection of conductor lines buried beneath a dielectric layer (30 .mu.m). Since it is always desirable to trim resistors with somewhat more energy than is necessary to simply remove the resistor material, so as to assure complete removal of all resistive paths, the dielectric must be expected to trim well without crack propagation.